Semiconductor devices currently in production or under commercial development, can be classified in three categories: single crystalline silicon, bulk polycrystalline silicon or thin film semiconductor. The first two are expensive to produce and are aimed at the high end of the market. Most research and development is aimed at improving the efficiency and long-term stability of such devices with little regard to cost.
On the other hand, thin film semiconductors are generally intended for lower cost applications, where absolute efficiency and compactness is not a prerequisite. Long-term stability is still a desirable goal for existing thin film semiconductors because of the relatively high production costs and the high material costs of the substrate.
Most commercial thin films semiconductor devices are based on hydrogenated amorphous silicon (a-Si:H) which may also contain nanocrystalline or microcrystalline phases deposited by various chemical vapour deposition (CVD) processes. The two main techniques are plasma enhanced CVD (PECVD) also know as glow discharge CVD and catalytic CVD (cat-CVD) also known as hot wire CVD (HWCVD). In the case of photovoltaic semiconductor devices, all CVD deposited thin film semiconductors and a-Si:H, in particular, suffer from light induced degradation (referred to as “the Staebler-Wronksi effect”) which is most significant over the first few years of operation. Consequently, all commercial photovoltaic semiconductor devices are over-engineered so that they degrade according to the specification and are consequently fitted with regulators to keep the output voltage constant. In commercial semiconduction based solar cells, the substrate is typically glass which limits the size of the semiconductor device due to its mechanical stability and weight. Glass also has the added disadvantage of being rigid and brittle.
All commercial thin film transistors (TFT) are based on CVD deposited semiconductor films and involve extensive use of photolithography, masks and chemical etching. In high density transistor arrays, a vertical stacking sequence of different semiconductor layers is used. The deposition techniques for depositing the various semiconductor layers, including for the gate electrodes, are the same as for semiconduction based solar cells and the usual substrate is glass.
A major factor limiting the production of inexpensive semiconductor devices, is the use of vacuum deposition processes in the production. Such vacuum deposition processes are relatively expensive and as such, add significantly to the cost of producing semiconductor devices. Further, semiconductor layers deposited using CVD processes are relatively rigid and as such, do not combine well with flexible substrates.